1. Field of the Invention
The present invention is directed to spacers for mounting fasteners in sandwich panels, a method of installing the spacers so that the spacers are flush with an upper surface of the panels and can provide a liquid tight seal and a resultant improved panel assembly, such as a floor of an aircraft.
2. Description of Related Art
In the aerospace field a premium is placed upon light-weight but strong structural components. For example, a floor of an aircraft must be lightweight and strong and also must accommodate the stresses imposed upon the bulkhead by the flexing of the wings during flight. The aircraft industry uses sandwich panels that are fastened to spars and bulkheads by a large number of fasteners that are secured to the panels by extending through corresponding spacers mounted in the panels. Frequently, the sandwich panels are formed of thin aluminum face plates with expandable aluminum foil strips to form a core or laminated plastic resin upper and lower surface face sheets that sandwich a honeycomb interior structure of resin and paper. Both configurations provide excellent strength to weight or stiffness to weight relationships in comparison to solid metallic panels. Alternative face sheets or skins for sandwich panels can include steel, titanium, magnesium, aluminum alloys, and alloy steels, while cores can also be formed of plastic foam, balsa wood, high temperature alloys, plastic syntactic and steel foil. Generally, the core is honeycomb and has hexagonal cells with walls perpendicular to the face sheets.
Composite panels are frequently used in aircraft with high-strength, high-modulus, fiber-reinforced, thermostat or thermoplastic resins. However, such fiber-reinforced composite panels do not necessarily respond well to localized concentrated loading forces. As can be appreciated, when a composite structure is used as a floor panel or wall panel in an aircraft, it is frequently necessary to fasten objects to the panels. Thus, a number of different fasteners and spacers have been developed to accommodate resin sandwich panels and to prevent a localized concentration of loading.
Some of the problems that have been recognized in the installation of a spacer are the necessary chemical compatibility of any fastener or spacer used in sandwich panels so that there is not any galvanic corrosion. This issue frequently arises when the sandwich panels contain carbon fiber, and if the fibers come in contact with a less noble metallic fastener, there can be corrosion, thus, magnesium, aluminum, aluminum alloys, and alloy steels are frequently not compatible with a graphite based structure.
Another problem that has occurred with mounting fasteners and spacers in a sandwich panel is that the panel does not have a significant transverse reinforcement. Thus, when a hole is drilled for mounting a spacer, the edge of the hole can be crushed, since there is relatively little resistance to a crushing force.
As can be appreciated, the application of a fastener or spacer to a sandwich panel can damage the panel in drilling holes in the panels in that surface ply splintering, surface ply delaminating, heat damage, and irregularities inside the hole can occur.
When composite panels are drilled or machined, the natural sealing process of lamination is disrupted. In areas where the fibers become exposed, an action can take place in which water, fuel, spilt liquids, etc. can be slowly absorbed through the fiber matrix interface into the structure. If the sandwich panels have an aluminum face sheet, the liquid can leak into the interior core of the panel. These leakage problems can result in weight gain, laminate degradation, and even unpleasant odors.
It is highly desirable that at most only a minimum portion of a fastener or spacer extend above the upper surface of a sandwich panel. One of the approaches in the prior art to address this issue has been to provide a rim in a spacer member that is mechanically locked by flaring the rim over the edge of the hole. Some installation of spacers in an aircraft floor sandwich panel have intentionally dimpled a metallic face skin where each spacer is to be inserted in order to insure a flush mounting. Another approach has been to provide a sleeve and plug composite spacer with a pair of flanges that overlap both sides of the hole. Any protrusions above the upper surface can be abraded and can further cause carpet wear. Adhesives can be applied to flanges to directly adhere a spacer member to the surface of a sandwich panel. In an attempt to seal an aperture in the spacer, ring seal washers have sometimes been used on fasteners in an attempt to sealingly compress them in the aperture. Additionally, a potting material of an appropriate epoxy can be used for further securing these types of spacers and fasteners.
The aerospace industry generally desires to automate the installation of spacers and fasteners to eliminate the high labor cost, wherever possible, particularly when hundreds and thousands of fasteners and spacers can be used in a commercial aircraft.
Thus, the prior art is still attempting to optimize the design of spacers and fasteners used in sandwich panels in aircraft and the problems of sealing with a flush installation of spacers with a surface of a panel has yet to be optimized.